This invention relates to a prosthesis for a joint and, more particularly but not exclusively to a partial hip prosthesis.
It is known that partial hip prostheses in general have a femoral part which includes a femoral head connected to a rod or a pin able to be engaged into the femur, the femoral head being designed to engage and pivot in the acetabulum in the manner of a ball-and-socket joint.
Such known prostheses are made either entirely of metal or of metal and plastic. In the latter case a spherical plastic cap is applied to the head of a metal femoral part.
It has become known that friction of a metal or plastic foreign body, such as results from use of these known prostheses, entails a number of disadvantages such as pain, wear of the cartilage, or even penetration by the inert material into the cartilage and into the subchondral bone.
Among known partial prostheses, there are some which have a cap not sealed into the pelvis and which is held on the spherical head of the prosthesis by a joint which renders the cap immobile. On the other hand, this cap is mounted fairly firmly on the spherical bearing surface of the femoral part of the prosthesis and most of the movement will occur, not between the inert parts of the prosthesis, but between the spherical cap and the cartilage.